Keeping abreast with the development of electronic industry, various data transmission interfaces, such as Universal Serial Bus (USB), External Serial ATA, e-SATA and the like, appear to the market one after another and are the common data transmission interfaces currently. Besides, because the storage capacity of Non-Volatile Memory (NVM) is significantly upgraded while the size thereof is reduced, portable data storage devices formed by combining data transmission interfaces and NVM have been commonplace recently, thereby further facilitating access and transmission of digital information of people. Above all, the use of flash drive is the most popular product among all. Although digitalization surely delivers more convenient life styles to all walks of life, it also adds more and more carry-on items in daily life of people inevitably. As a result, more lost belongings and difficult storage are followed next.
With reference to FIG. 1, a flash drive taking the form of a pen and disclosed by Taiwanese Patent Publication No. M248624 combines a pen housing (1), an USB connector (2) and a memory device (3) for users to carry so as to reduce the chance of lost belongings and tackle difficult storage. However, due to the standardized specifications of the USB connector (2) and the size of the memory device (3), the outer diameter of the pen housing (1) becomes oversized if the USB connector (2) and the memory device (3) are received therein. Thus, the oversize pen housing (1) makes writing with the pen inconvenient and fails to comply with the call for products being light, thin and short. Therefore, under the premise of less lost belongings and easier storage, how to maintain existing performance and size of conventional products and avail to mount data storage device is certainly a critical subject.